Some projectiles are not being slowed down by cobweb.
This bug was fixed in snapshot 15w49a, but as a result it was possible to use witches and snow golems to get unlimited items(MC-93685) and caused another bug(MC-93636) that removed the feature, which should only slow down projectiles instead of turning them into items, like bubble columns(MC-125757).
Affected projectiles:
arrow
spectral_arrow
trident
egg
snowball
ender_pearl
potion
fireball
small_fireball
dragon_fireball
shulker_bullet
experience_bottle
llama_spit
wither_skull
firework_rocket
fishing_bobber
wind_charge
breeze_wind_charge
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Can confirm in 24w03a